The present invention relates to an apparatus for power control by phase gating of an AC voltage, which supplies an electrical load, and for reduction of harmonics which are created by the phase gating up to a region of 4 kHz, preferably in the region of the third harmonic, having an electric motor as the load, having a first circuit element which is connected in series with the load and is driven by a control device in order to carry out phase gating, and having a resistance element. The invention also relates to a method for harmonic reduction in the range up to 4 kHz, preferably in the region of the third harmonic, for power control by phase gating.
An apparatus and a method for power control by means of phase gating are known, for example, from the article by S. Williamson et al. “Acoustic noise and pulsating torques in a TRIAC-controlled permanent-split-capacitor fan motor”, IEE Proceedings, Vol. 128, Pt.B, No. 4, July 1981, pages 201-206, or the application DE 198 50 905 A1 from the same applicant.
Document DE 10052910 A1 discloses an electric machine which comprises a brake resistance directly mounted on the machine. DE 2 131 750 A discloses a circuit for reducing distortions caused by phase gating. The circuit comprises to TRIACs which are triggered in succession. EP 0 760 551 A2 discloses an electrical device for controlling heater. The device also comprises two TRIACs for switching the heater.
Generally apparatuses for phase gating are used to control the power which is supplied to an electrical load by periodically switching the load on and off via a circuit element. A TRIAC is normally used as the circuit element, is connected in series with the load, and is triggered via a control device. The control device has a series circuit comprising a resistor and a trigger capacitor, which is arranged in parallel with the TRIAC. The trigger signal is tapped off between the resistor and the trigger capacitor and is supplied to the control input (gate) of the TRIAC via a series circuit comprising a resistor and a trigger element in the form of DIAC. In addition to these control devices, which are designed in analog, integrated circuits have also in the meantime become available which carry out this drive function.
In the case of power control by phase gating of the AC voltage that is supplied to the load, undesirable harmonics are produced, whose extent is governed by European Standards. These Standards must be satisfied by the power control apparatuses, for which reason solutions in order to reduce these harmonics have been proposed in recent years.
The third harmonic in particular plays a special role in the reduction of harmonics, for which reason previous solutions have concentrated in particular on the reduction of these third harmonics. One example of an apparatus for reducing the third harmonic is disclosed, for example, in EP 0 859 452 B1 from the same applicant. In the solution disclosed there, the phase angle or trigger angle is varied by the control device by a predetermined value, for example in successive full cycles. The result of such asymmetry in the trigger angle leads to even-numbered harmonics which increase only slowly, and to greatly reduced odd-numbered harmonics.
Although this solution approach has been proven in practice and allows the existing Standards to be complied with in this way without any problems, there is, of course, still the desire to find an even more cost-effective solution. Furthermore, an apparatus would be desirable whose humming in the region of 25 Hz is considerably less than in the case of the apparatus disclosed in the abovementioned document.